It is common practice in the manufacture of thermoplastic automotive interior trim panels to form the cover or skin for the panel by casting very small particles of a selected thermoplastic polymer in a wet or dry casting process against a heated shell mold whose molding surface is typically formed with a grained texture. The tool has a thin nickel coating which imparts a desired grained texture on the appearance surface of the molded panel. These small thermoplastic particles are commonly formed with an underwater pelletizer die system wherein molten polymer of the desired type is extruded through a plurality of extruding holes of uniform size in an extruding die, is cooled under water and thereby solidified by circulated cooling water. As it exits the extruding holes, the thermoplastic is cut into small particles which are carried off with the water for collection. The small thermoplastic particles thus produced are commonly called Minibeads.TM. pellets and are not perfectly round spheres nor do they necessarily have an extruded diameter exactly equal to the die hole size. The three dimensional size of the pellets is controlled by the size of the die hole, the speed of the cutter relative to the rate of polymer extrusion, and the die swell of the polymer. The pellets are commonly described as having a certain single dimensional size which is their maximum cross section dimension and which is the pellet dimension referred to herein.
For making thermoplastic pellets for the casting of an automotive interior trim article, the extruding die in such a pelletizer is normally provided with a very large number of extruding holes (e.g. 240) that are all of the same size; i.e. produce pellets of a narrow distribution. For example, the polymer pellets that are currently commercially available for molding automotive interior trim panels in a casting process are produced in this manner and with different extruding dies down to a size of 0.020 inches. The 0.020 inch pellets have been found to cast well and produce excellent results provided there are no tight returns or corners or very fine surface details in the article being molded. Where there are tight returns and/or corners and/or very fine surface details which is often the case in automotive interior trim panels, the minibeads may not seat in these areas on the mold surface resulting in porosity, unacceptable cavities or pits in the appearance surface of the molded panel at these locations.
This problem can be avoided by making all the pellets of a suitable smaller uniform size that will seat in the most difficult areas on the mold surface. For example, extruding die holes as small as 0.008 inches may be readily formed in an extruding die using laser machining. It has been found that pellets about this size will seat with excellent results on a mold surface having the tightest returns, corners and surface features normally encountered in an automotive trim panel. But this much smaller size pellet significantly slows the rate of production of the pellets, requiring a much greater number of pellets for a particular cast part, and consequently significantly increases the cost to produce the cast part. For example, the production output of pellets is decreased by a factor of 8 by weight in an underwater pelletizer when the pellet or bead size is halved and the same number of extruding holes is used in the extruding die. And even by doubling the number of extruding holes in the die, the pellet production is still reduced by a factor of 4 by weight as compared with pellets twice the size produced with half the number of extruding holes.